This invention relates to the preparation of nitrogen containing films on substrates and more particularly to preparation of nitride films on semiconductors by introducing nitrogen into a corona discharge, thereby to produce activated nitrogen molecules for application to the substrate.
Rapid progress has been realized in semiconductor technology related to the growth of group III metal-nitride films (III-N). Metal-organic vapor-phase epitaxy (MOVPE) has emerged as the leading xe2x80x9cIII-Nxe2x80x9d growth process, generally utilizing ammonia and trimethylmetallics as the precursors for nitrogen and group III metals, respectively. In contrast, and despite potential advantages over MOVPE, progress in molecular beam epitaxy (MBE) of III-N semiconductors has been slow. MBE offers a pristine UHV growth environment, a precise control over layer-by-layer composition, the ability to monitor growth in situ via standard surface science tools, and a wide choice of sources. For III-N growth, the latter include effusive or supersonic jets of MOVPE species, discharge sources of activated nitrogen gas, and evaporation sources of high purity metals.
AlN, GaN, and InN are emerging as materials of choice for wide band gap semiconductor devices. Despite the remarkable advances in III-N fabrication, further improvements of growth processes are needed.
In accordance with this invention, certain electronically-excited (or xe2x80x9cactivatedxe2x80x9d) nitrogen molecules are used as an ideal nitride precursor. This invention provides a method of producing N2A3xcexa3u+ in essentially a pure state as the sole excited species in a molecular beam whose only other component is ground state nitrogen. Through the use of this method, a high purity, high energy form of nitrogen is produced that when used in III-N fabrication, greatly improves upon both the quality and consistency of metal-III nitride and its production control. This method enables metal-III nitride film production to take advantage of the above discussed benefits of MBE. (The terms xe2x80x9cfilmxe2x80x9d and xe2x80x9clayerxe2x80x9d are used interchangeably herein with no distinction intended.)
A corona discharge supersonic free-jet (CD-SFJ) source has been constructed, characterized, and operated to grow III-N nitride semiconductor films via molecular beam epitaxy (MBE) using A3xcexa3u+N2, an electronically-excited metastable molecule. By xe2x80x9cmetastablexe2x80x9d is meant molecules having a lifetime in their activated state sufficient to bring them to a location where nitride film growth occurs. The CD-SFJ yields A3xcexa3u+ molecules as the sole activated species in a molecular beam otherwise containing only X1xcexa3g+ ground state nitrogen molecules plus a negligible quantity of 4S0 ground state nitrogen atoms. The generated beam has been used as a nitrogen source to epitaxially grow metal-nitride (III-N) films via this rich content of excited metastable nitrogen molecules. Optical emission spectroscopy of the free-jet expansion reveals the expected cascade through the excited state manifold of N2 triplet states to populate the A3xcexa3u+ metastable state. Appearance potential spectroscopy (mass spectrometer detector ionization yield, measured as a function of electron impact energy) explicitly establishes the fraction of all activated nitrogen species in the terminal beam, including non-emitting metastable species such as the A3xcexa3u+ state. Metastable A3xcexa3u+ molecules are present at up to 1.6% number fraction even several meters from the source, providing a beam intensity of 8.5xc3x971016 metastables srxe2x88x921sxe2x88x921. Growth studies confirm that A3xcexa3u+ does incorporate very efficiently into a growing GaN thin film.
The electronically excited A3xcexa3u+ nitrogen molecules couple to the ground state solely via the forbidden Vegard-Kaplan bands. The A3xcexa3u+ lifetime is therefore very long, circa one second, and far greater than beam transit times through a molecular beam apparatus. Consequently, A3xcexa3u+ molecules are metastable insofar as molecular beam chemistry is concerned, and can be employed in molecular beam epitaxy (MBE) in the fashion of any stable beam species. The suggested use of N2A3xcexa3u+ to grow GaN is based on two facts: (1) being an electronically excited state, A3xcexa3u+ is reactive (xe2x80x9cactivatedxe2x80x9d), and (2) being molecular rather than atomic, A3xcexa3u+ delivers two atoms simultaneously to the surface. Thereby, in a dissociative chemisorption reaction, one of these two atoms can bind to the surface while the second carries away the heat of reaction as kinetic energy. As a result, the strong exothermicity of the III-N reaction need not be dissipated through the growing III-N film. Nitrogen accommodation is enhanced and sputtering damage minimized, to yield high quality films at a growth rate limited only by the flux at which the metastable molecules are supplied to the film.
Many different varieties of plasma sources, often incorporating supersonic jet techniques, have been employed to xe2x80x9cactivatexe2x80x9d nitrogen for III-N growth. These have included radio frequency (RF) discharges, microwave discharges, electron cyclotron resonance (ECR) discharges, various arc-jet discharges, and hollow anode plasma discharges. Invariably, these sources produce a broad spectrum of both atomic and molecular excited states and often ionic states as well. In marked contrast, a corona discharge supersonic free-jet expansion can yield predominantly the long-lived metastable N2A3xcexa3u+ state and in appreciable quantities. As such it becomes the primary candidate for testing GaN growth via A3xcexa3u+ nitrogen molecules.
The above and further objects and advantages of the invention will be better understood with reference to the following Detailed Description taken in consideration with the accompanying drawings.